reddit: We are nuclear fusion researchers, ask us anything

[KitemanSA]

I was under the impression that the high initial cost of a tok plus the fact that neutrons destroy its own equipment in a matter of months makes the approach economically infeasible, even if the reaction yields surplus energy. No utility in its right mind would invest in one.

At least fission reactors work suffering high neutron flux. Some of them produce 1kW*h at 2-3 cents of cost.

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Be noted that fission neutron average kinetic energy 3 times higher than for neutrons produced by DT reaction. But at the same power fusion reactors need 3 times higher flux. This is solvable problem. And about ten different types of Tritium Breading Modules TBM are already ready for testing for ITER.

The distinction of course is that in a fission reactor, the neutrons are released withing very dense fuel solids or liquids and generally get deposited into onther fuel elements while with fusion, all the neutrons generally get deposited into the walls.

[Robthebob]

This is from one of my professors who's a dusty plasma physicist

"Robert, I would consider there to be three main problems: a) micro instabilities remain a major problem. They cause localized enhanced transport, but their scale lengths and lifetimes are often too short to allow active feedback control. b) this is then connected to the second problem - which is confinement. The instabilities cause the confinement of the plasma to be reduced, which limits the system performance. c) the third issue is newly emerging as the previous two are now better defined - this is the plasma-wall interactions. This is the critical issue for turning a fusion experiment into a fusion power plant - how to interface a fully thermonuclear plasma environment with a physical wall through which you have to extract power. As a colleague of mine put it, there are no elements on the period chart that can stand up to a burning plasma!"

I guess if you want a physics problem, there's one, there are no physical material that can deal with a burning plasma. As for the H mode... it's not very well understood, apparently the neutron injection drives plasma flow, which in turns may drive the system into H mode, but I dont think what you suggested, which is that neutron injection generates the plasma current, is right... at all.

the plasma current is induced by the changing magnetic field.

[Joseph Chikva]

This is from one of my professors who's a dusty plasma physicist

"Robert, I would consider there to be three main problems: a) micro instabilities remain a major problem. They cause localized enhanced transport, but their scale lengths and lifetimes are often too short to allow active feedback control. b) this is then connected to the second problem - which is confinement. The instabilities cause the confinement of the plasma to be reduced, which limits the system performance. c) the third issue is newly emerging as the previous two are now better defined - this is the plasma-wall interactions. This is the critical issue for turning a fusion experiment into a fusion power plant - how to interface a fully thermonuclear plasma environment with a physical wall through which you have to extract power. As a colleague of mine put it, there are no elements on the period chart that can stand up to a burning plasma!"

I guess if you want a physics problem, there's one, there are no physical material that can deal with a burning plasma. As for the H mode... it's not very well understood, apparently the neutron injection drives plasma flow, which in turns may drive the system into H mode, but I dont think what you suggested, which is that neutron injection generates the plasma current, is right... at all.

the plasma current is induced by the changing magnetic field.

Robert, your professor is wrong as real 2-3 seconds of confinement time was achieved on JET TOKAMAK in Culham, UK. The goal of ITER is to achieve of about 1000 sec of confinement time.
And H-mode may be not well understood for him - specialist in dusty plasma and not in fusion. As he gives you 70s of last century data.
Not "neutron" but "neutral injection". And this injection really drives plasma current when possibility to apply an inductive voltage ends.

[Joseph Chikva]

I was under the impression that the high initial cost of a tok plus the fact that neutrons destroy its own equipment in a matter of months makes the approach economically infeasible, even if the reaction yields surplus energy. No utility in its right mind would invest in one.

At least fission reactors work suffering high neutron flux. Some of them produce 1kW*h at 2-3 cents of cost.

Wahh ha ha ha ha ha ha,
yee ha ha ha hoho ha
Hee hee hee haw haw ho ho
WAAA ha ha ha ha...

At least nuke plant located in nearby for me Armenia sells 1kW*h at 3 cents. This is a fact.

[ladajo]

Area under the curve.

Sorry, I want to correct myself. Volume under the surface.

What unit that volume has: m3 or m3*sec? Surface is 2 or 3 dimensional?

Actually the surface is 4D. At any given instant it is 3D. And that can provide for instantaneous analysis. However, the machine must be analyzed for not only where things are, but where they were, and where they will be. Since it is a 4D chaotic construct, this makes for some fun math. We still don't even fully understand the bounds, and in an operating steady state Polywell machine the plasma bounds will actually be somewhat dynamic. The trick is that the entire soup is dynamic within a total bounds.
My point in this to you is that it is very unlikely that any particle will track a straight path at any time in the machine. By design it is messy. There have been some very interesting studies done on fusors (and polywell) exploring the distributions and tendancies. Polywell was never meant to be stable, it is meant to concentrate. Think of it as an angry jello ball trying to expand and escape. But the magnetic fist keeps squeezing it. The trick is in how tight the fingers can grasp the slippery expanding jello ball and keep it from squirting out. A tricky balance for sure, but not impossible.

[Joseph Chikva]

I thought we were talking about fusion. Not space programs.

I am saying that one commercial Tokamak WILL cost billions (and billions).

I am also saying that other approaches may well pan out to be orders of magnitude cheaper to build commercial plants.

No, I am talking about the cost of any type of program - does not matter: fusion or space or military. Or in the other words - any labor intensive program.
When thousands PhD are involved and when each of them has annual salary equal to 100'000 USD, you have salary annual expenses hundreds of millions dollars. And if that program lasts 30 years, the total expenses on only salary - several billions.
Plus trips. seminars, workshops, etc.
So, 16 or 20 billions have to be spent for ITER development is not the cost of hardware.

[Joseph Chikva]

Sorry, I want to correct myself. Volume under the surface.

What unit that volume has: m3 or m3*sec? Surface is 2 or 3 dimensional?

Actually the surface is 4D. At any given instant it is 3D. And that can provide for instantaneous analysis. However, the machine must be analyzed for not only where things are, but where they were, and where they will be. Since it is a 4D chaotic construct, this makes for some fun math. We still don't even fully understand the bounds, and in an operating steady state Polywell machine the plasma bounds will actually be somewhat dynamic. The trick is that the entire soup is dynamic within a total bounds.
My point in this to you is that it is very unlikely that any particle will track a straight path at any time in the machine. By design it is messy. There have been some very interesting studies done on fusors (and polywell) exploring the distributions and tendancies. Polywell was never meant to be stable, it is meant to concentrate. Think of it as an angry jello ball trying to expand and escape. But the magnetic fist keeps squeezing it. The trick is in how tight the fingers can grasp the slippery expanding jello ball and keep it from squirting out. A tricky balance for sure, but not impossible.

I a little bit aware with magnetic confinement. But your vision on geometry is very new for me. As by my understanding if closed surface limits some nD volume, dimension of that surface should be (n-1). Thanks.
I think that talking about space-time Einstein had a little different thoughts than you.

[Joseph Chikva]

The distinction of course is that in a fission reactor, the neutrons are released withing very dense fuel solids or liquids and generally get deposited into onther fuel elements while with fusion, all the neutrons generally get deposited into the walls.

Hehe. And what? Would you like to say that first aneutronic fusion reactors will be built? First wall issue is a challenge, but solvable challenge.

[Joseph Chikva]

the plasma current is induced by the changing magnetic field.

Not only.
http://iopscience.iop.org/0029-5515/49/ ... 042001.pdf

Effect of plasma rotation on the
beam-driven current
G.A. Cottrell and R. Kemp
EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon,
OX14 3DB, UK

Received 26 January 2009, accepted for publication 23 February 2009
Published 10 March 2009
Online at stacks.iop.org/NF/49/042001
Abstract
In a rotating plasma, with co-neutral beam injection (NBI), the Doppler shift of the NBI particles, as viewed in the
frame of the plasma, can result in a significant reduction in the beam-driven (Ohkawa) current when the rotation
is strong (i.e. with rotational Mach numbers, M 0.5). The correction applies to the toroidal fast ion current
calculated for a non-rotating plasma and is independent of the normal Zeff and electron trapping terms. A simple
analytical model is presented to estimate the magnitude of the effect for plasmas with arbitrary toroidal rotation and
the conditions where this is important have been identified. This model has been compared with the results from
existing Monte Carlo neutral beam codes and found to reproduce their results. The important parameters in this
problem are the ratio, ρLab = vLab
f 0 /vcrit , of the NBI injection particle velocity (in the laboratory frame) to the critical
velocity of the plasma, and the ratio ρφ = vφ/vcrit which is related to the rotational Mach number. A phase plot
in dimensionless (ρLab, ρφ) space is presented which enables the fast ion current drive efficiencies to be compared
for different tokamaks. For strongly rotating plasmas, the degradation in fast ion current efficiency is significant for
ρLab 1. However, when ρLab is larger than this, the degradation in fast

[KitemanSA]

At least fission reactors work suffering high neutron flux. Some of them produce 1kW*h at 2-3 cents of cost.

Wahh ha ha ha ha ha ha,
yee ha ha ha hoho ha
Hee hee hee haw haw ho ho
WAAA ha ha ha ha...

At least nuke plant located in nearby for me Armenia sells 1kW*h at 3 cents. This is a fact.

My apologies. Here we were, talking FUSION and you chime in with a statement about fission. I misread.

Yes, I do think that subsidized FISSION may be available at a few cents/kWh. Indeed, in bulk amounts, even UNsubsidized fission may approach that value.

But yet again, you adroitly jumped from the topic (FUSION) being commercializable and when, to a totally OFF TOPIC statement about FISSION. Can you PLEASE pick a topic and stick with it?

[Joseph Chikva]

But yet again, you adroitly jumped from the topic (FUSION) being commercializble and when, to a totally OFF TOPIC statement about FISSION.

You are wrong. I only said that we have an example of exploiting reactors suffering high neutron flux and producing energy at acceptable cost. At least in most cases renewable energy is more costly.

And what's up with fusion?

[Robthebob]

Sorry, I was wrong about the word neutral and neutral beam driven plasma current.

[KitemanSA]

Lets bring things back into focus.

Be noted that fission neutron average kinetic energy 3 times higher than for neutrons produced by DT reaction. But at the same power fusion reactors need 3 times higher flux. This is solvable problem. And about ten different types of Tritium Breading Modules TBM are already ready for testing for ITER

.

The distinction of course is that in a fission reactor, the neutrons are released withing very dense fuel solids or liquids and generally get deposited into onther fuel elements while with fusion, all the neutrons generally get deposited into the walls.

Hehe. And what? Would you like to say that first aneutronic fusion reactors will be built? First wall issue is a challenge, but solvable challenge.

YOU compared fission neutrons and ITER neutrons, not fission and aneutronic or fission and LENR or anything else. YOU compared fission and ITER.
Well, I commented about fission versus fusion and stated that the prime problem is that with fusion, the neutrons get deposited in the walls. Again, with a tad of shorthand and generalization, fission vs ITER, or other DT fusion which you also mentioned.

I pointed out this fact and then you snidely bring up "a-neutronic" fusion. Well, the neutrons with those reactions are ALSO depositied in the walls. If you think there are none, think again.

Yet again, you make a statement, get called on it, and skittle all over the floor avoiding the topic while "hehe"ing others.

Now, to your specific statement "First wall issue is a challenge, but solvable challenge", true, and much MORE EASILY solvable with fission or a-neutronic fusion like Polywell MAY become and ITER will NEVER become.

Polywell MAY become commercial, toks almost certainly will not.

[KitemanSA]

But yet again, you adroitly jumped from the topic (FUSION) being commercializble and when, to a totally OFF TOPIC statement about FISSION.

You are wrong. I only said that we have an example of exploiting reactors suffering high neutron flux and producing energy at acceptable cost. At least in most cases renewable energy is more costly.

If your fission reactor walls are suffering high neutron flux, maybe you need better designers. But now comes the argument of what "high" means. Just watch.

[Joseph Chikva]

Lets bring things back into focus.

Be noted that fission neutron average kinetic energy 3 times higher than for neutrons produced by DT reaction. But at the same power fusion reactors need 3 times higher flux. This is solvable problem. And about ten different types of Tritium Breading Modules TBM are already ready for testing for ITER

.

The distinction of course is that in a fission reactor, the neutrons are released withing very dense fuel solids or liquids and generally get deposited into onther fuel elements while with fusion, all the neutrons generally get deposited into the walls.

Hehe. And what? Would you like to say that first aneutronic fusion reactors will be built? First wall issue is a challenge, but solvable challenge.

YOU compared fission neutrons and ITER neutrons, not fission and aneutronic or fission and LENR or anything else. YOU compared fission and ITER.
Well, I commented about fission versus fusion and stated that the prime problem is that with fusion, the neutrons get deposited in the walls. Again, with a tad of shorthand and generalization, fission vs ITER, or other DT fusion which you also mentioned.

I pointed out this fact and then you snidely bring up "a-neutronic" fusion. Well, the neutrons with those reactions are ALSO depositied in the walls. If you think there are none, think again.

Yet again, you make a statement, get called on it, and skittle all over the floor avoiding the topic while "hehe"ing others.

Now, to your specific statement "First wall issue is a challenge, but solvable challenge", true, and much MORE EASILY solvable with fission or a-neutronic fusion like Polywell MAY become and ITER will NEVER become.

Polywell MAY become commercial, toks almost certainly will not.

Long text.
For your reference if fuel in fuel rod suffering neutron flux, so, that also suffers changes in his physical properties.
If you have good designer, he also considers solid fuel as an structural element.
"High" flux is higher then "low"
If you want numbers, somewhere I have the book written in 70s, in which is written how many 14.1MeV neutrons is allowed by that (e.g. perlitic stainless steel) or another (e.g. zirconium) material per sq. m.
Now for ITER and as I know for NIF as well is suggested stainless steel combination with copper and beryllium. Do you know how that material works?
I am sure that better then perlitic stainless steel.
In any case by increasing if walls radial dimension you will decrease flux as square.
Polywell at first should show his viability on easy for realization neutrnic DT reaction. And that is the problem that Polywell did not show anything promising. And TOKAMAK did.
That is the difference.
I know too that better to be healthy than to be ill. And aneutronic better that neutronic. But let's at first you make neutronic.